Tumor necrosis factor (TNF) related apoptosis-inducing ligand (TRAIL, or Apo2L or TNFSF10) is the third apoptosis factor in the TNF family found after TNF and FasL, and is believed to be a promising anticancer biological medicine. TRAIL was cloned from myocardial cDNA library by Wiley et al., and was named not only because its amino acid sequence had the structure characteristics of the TNF superfamily, but also because it is capable of inducing apoptosis of Jurkat cells and EB virus-transformed human lymphocytes. TRAIL belongs to type II transmembrane protein and is composed of 281 amino acids. Its N terminal (1st-17th amino acids) is located inside the cell and its C terminal (39th-281st amino acids) extends outside the cell, wherein the 114th-281st amino acids play the main function.
A lot of preclinical researches have shown that TRAIL is capable of inducing apoptosis of various types of cancer cell lines with no side effects on normal cells. By now, phase I and II clinical trials of TRAIL and its receptor-agonist antibody have been conducted abroad, and preliminary efficacy has been achieved. Besides, TRAIL only exhibits very weak activation effect on NF-κB, therefore, even being administered systematically, it will not induce severe inflammatory response just as that produced by TNF-α and Fas-L. As a result of this, TRAIL has potential to develop into a new generation of anti-tumor medicine.
Located on cell membrane, TRAIL is expressed in the cells of immune system, including the NK cell, T cell, macrophage and dendritic cell, which can be processed into a soluble form by cysteine protease. TRAIL plays an apoptosis—inducing role by binding with its cell membrane receptors. Up to now, 5 kinds of TRAIL receptors have been found, including TRAIL-R1 (DR4, TNFSF10a) and TRAIL-R2 (DR5, TNFRSF10b); TRAIL-R3 (DcR1, TNFRSF10c) and TRAIL-R4 (DcR2, TNFRSF10d); and circulating osteoprotegerin (OPG, TNFRSF11b). DR4 and DR5 have a segment of death domain (DD), which is indispensable for inducing apoptosis after TRAIL binds with the receptors. Due to lack of functional death domain (DD), other 3 receptors, although being capable of binding with TRAIL, cannot induce apoptosis.
Binding of TRAIL with DR4 or DR5 may activate both mitochondrial-dependent and non mitochondrial-dependent apoptosis signaling pathways, mediate death signal to be transmitted into the cells, initiate the effector Caspase-3, and eventually induce tumor cell apoptosis.
Recently, it has been found that recombinant soluble TRAIL induces apoptosis of a broad spectrum of human tumor cell lines. However, there are still certain of tumor cells with low sensitivity or resistance to TRAIL, e.g., all the human melanoma cell lines, most breast cancer cell lines, prostate cancer cell line LNcaP and the like. Although DR4 or DR5 is more or less expressed on the surface of these cells, binding of TRAIL with DR4 or DR5 cannot induce the final apoptosis of the cells because of the absent or mutation of the key enzyme involved in the apoptosis pathway of the cells and the high expression level of other apoptosis inhibiting proteins.
For the solid tumor formed by these cells, alternative therapeutic means are needed urgently, or, other aspects, e.g., modifying TRAIL to make it able to kill the tumor cells with low sensitivity or resistance, are also need. As shown in studies, combined use of soluble recombinant TRAIL (rTRAIL) with radiotherapy and chemotherapy may increase sensitivity of tumor cells to rTRAIL, i.e., the two display a synergistic effect. Now, the internationally applied rTRAIL is the fragment of the 95th-281st amino acids. The rTRAIL monomer tends to form into a trimer having better biological activity, the top of which has a binding site of zinc ion that plays an important role on stabilizing the trimer's conformation.
Combined treatment protocol just includes administrating rTRAIL with these drugs simultaneously, but the link between them has not been set up, resulting in the failure of transporting these drugs to the tumor cells directionally. Thus, only drugs with low-dose and low-toxicity are usually selected, so as to prevent normal cells from damage. Accordingly, better efficacy cannot be achieved. Efficient utilization of strong tumor cell-killing drugs would be more helpful for tumor therapy. For example, MMAE (Monomethyl auristatin E), known as a chemotherapeutic drug, is a synthetic anti-tumor small molecule, which induces apoptosis by inhibiting dimerization of tubulin in cells. However, due to its highly unspecific toxicity, MMAE can damage normal cells. Hence, it is impossible for MMAE itself to be developed into a medicine. Besides, the studies on mutants of TRAIL and antibodies of TRAIL receptors are still in progress, but the efficacy thereof is not satisfactory.